This invention relates generally to the field of combustion turbines, and more specifically to a gas turbine including a catalytic combustor, and in particular to a passively cooled catalytic combustor having improved protection against overheating and a wider operating range.
In the operation of a conventional combustion turbine, intake air from the atmosphere is compressed and heated by a compressor and is caused to flow to a combustor, where fuel is mixed with the compressed air and the mixture is ignited and burned. This creates a high temperature, high pressure gas flow which is then expanded through a turbine to create mechanical energy for driving equipment, such as for generating electrical power or for running an industrial process. The combustion gasses are then exhausted from the turbine back into the atmosphere. Various schemes have been used to minimize the generation of pollutants during the combustion process. The use of catalytic combustion is known to reduce the generation of oxides of nitrogen since catalyst-aided combustion can occur at temperatures well below the temperatures necessary for the production of NOx species.
FIG. 1 illustrates a prior art gas turbine combustor 10 wherein at least a portion of the combustion takes place in a catalytic reactor 12. Compressed air 14 from a compressor (not shown) is mixed with a combustible fuel 16 supplied through fuel injectors 18 upstream of the catalytic reactor 12. Catalytic materials present on surfaces of the catalytic reactor 12 initiate the heterogeneous combustion reactions at temperatures lower than normal ignition temperatures. However, for certain fuels and engine designs such as natural gas lean combustion, known catalyst materials are not active at the compressor discharge supply temperature. A preheat burner 20 is provided to preheat the combustion air 14 by combusting a supply of preheat fuel 22 upstream of the main fuel injectors 18. One such system is described in U.S. Pat. No. 5,826,429 issued on Oct. 27, 1998, incorporated by reference herein. Such pre-burn systems are costly and they add complexity to the design and operation of the combustor.
The surface reactions within the catalytic reactor release enough heat energy to cause auto-ignition and combustion of the remainder of the fuel in the gas stream beyond the catalytic reactor 12, in a region of the combustion chamber called the burnout zone 24. For modern high firing temperature combustion turbines, the amount of fuel reacted in the catalyst bed must be limited in order to prevent overheating of the materials within the reactor. In order to cool the catalytic reactor 12 and to limit the amount of conversion within the reactor, it is known to provide both catalyzed and non-catalyzed substrate passages through the catalytic reactor 12. Such designs are described in U.S. Pat. No. 4,870,824 dated Oct. 3, 1989, and U.S. Pat. No. 5,512,250 dated Apr. 30, 1996, also incorporated by reference herein. The fuel-air mixture passing through the non-catalyzed passages serves to cool the catalytic reactor 12 while retaining the removed heat in the combustion gas stream. While such passive cooling is an improvement over previous designs, there remains a risk of the fuel-air mixture in the non-catalyst cooling passages igniting or of the flame traveling upstream into the non-catalyzed cooling passages. In such an event, the cooling action will be lost and the catalyst may overheat and fail.
Accordingly, an improved catalytic combustor is needed to reduce the risk of overheating of the catalytic reactor. Furthermore, a simple and cost effective catalytic combustor is needed for applications where the gas supply temperature is below the temperature necessary to activate the catalyst.
A combustor is described herein as having: a heat exchanger module having catalytic passages in a heat exchange relationship with non-catalytic passages; a fuel injection apparatus; and a means for directing combustion air in sequence through the non-catalytic passages, the fuel injection apparatus and the catalytic passages. Because the air traveling through the non-catalytic passages does not contain fuel, the risk of flash-back of the flame into these cooling passages is eliminated.
In one embodiment, a combustor is described herein as including: a plurality of catalyst modules disposed in a generally circular pattern at the inlet of an annular combustor chamber within an engine casing; a seal between the plurality of catalyst modules and the engine casing for directing a flow of air into contact with non-catalytic surfaces of the respective catalyst modules; a plurality of fuel injectors associated with the plurality of catalyst modules for injecting a combustible fuel into the flow of air downstream of the non-catalytic surfaces to form a fuel-air mixture; and a plurality of catalytic surfaces formed on the catalyst modules for contacting the fuel-air mixture downstream of the non-catalytic surfaces and for causing a first portion of the fuel to combust within the respective catalyst modules and a second portion of the fuel to combust within the combustion chamber.
A gas turbine is described herein as including: a compressor for providing a flow of air; a combustor for combusting a flow of fuel in the flow of air to produce a flow of combustion gas; and a turbine for extracting energy from the flow of combustion gas; wherein the combustor further comprises: a catalyst module having a catalytic surface and a non-catalytic surface in heat exchange relationship there between; a fuel delivery apparatus; and a flow directing apparatus for directing the flow of air in sequence from the non-catalytic surface to the fuel delivery apparatus to the catalytic surface.
A method of combusting a fuel is described herein as including the steps of: providing a catalyst device having a catalytic surface in heat exchange relationship with a non-catalytic surface; directing fuel-free air over the non-catalytic surface to remove heat energy from the catalyst device and to pre-heat the fuel-free air; adding a combustible fuel to the fuel-free air to form a fuel-air mixture; and directing the fuel-air mixture over the catalytic surface to combust at least a first portion of the fuel-air mixture and to generate heat energy.